The present invention is related to wireless networks, and in particular to radio plan generation to determine transmit frequencies and transmit powers in a wireless local area network (WLAN).
Radio plan generation for wireless local area networks, including the selection of a frequency channel, antenna configuration, and a transmit power level for each of a potentially large number of access points (APs), is an integral radio management function of a WLAN system. Current technology successfully manages in the order of 10's of APs, developing the radio plan in a timely fashion. Consider, for example, an IEEE 802.11b network that includes 10 typical access points. Each access point can be assigned to one of three possible channels and one of six possible power levels. Thus each access point has 18 possible configurations. This network would therefore have 1810 or approximately 3.6 trillion possible configurations. An exhaustive search of each possible configuration would take an extremely long time considering that some sort of quality metric must be evaluated for each considered configuration. Thus there is a need to develop a plan rapidly.
Furthermore, the number of APs to be managed is likely to grow significantly in the future, as could be the case in the deployment of a high-density enterprise system. It is desirable to have a technique that scales more directly to the number of APs.
Timeliness of frequency assignment is also a strong driver for implementation of dynamic frequency assignment standards, such as the IEEE 802.11 h standard, wherein mandated frequency changes can drive reassignment of large groups of APs. This must be done on the fly, with as little impact as possible on network throughput. Speed of reassignment becomes a key factor, especially when a large number of APs is involved.
Radio planning is known for cellular telephones, in particular, for so-called second generation (2G) cellular telephony. Many methods are known for frequency allocation for 2G cellular telephony. See for example,
I. Katzela, N. Naghshineh; Channel assignment schemes for cellular mobile telecommunication systems: a comprehensive survey; IEEE Personal Communications (June '96).
See also, U.S. Pat. No. 6,023,459 to Clark et al., and U.S. Pat. No. 6,178,328 to Tang, et al. These methods assign frequency channel, but do not simultaneously assign transmit power.
In 2G cellular telephony, devices in each cell transmit without regard to who is transmitting in other cells. Communication is still reliable as long as the desired signal power is sufficiently far above the total interference power from all the other devices on the same channel. Therefore, in 2G systems, every effort is made to reduce the interference power. As a first step, channels are not reused until distant cells are available for such channels. This can be expressed as a graph coloring problem, where colors represent frequencies, and according to which physically adjacent (or more generally, nearly adjacent) regions have to be colored differently.
WLANs have much smaller cells than 2G systems, and hence achieve much higher per-user data rates. In exchange, WLANs cannot rely on regular cells, nor can the tricks of 2G engineers make up the difference. Walls, windows, doors, partitions, ceilings, and even filing cabinets can lead to anomalous propagation and non-uniform or overlapping cells.
For this and other reasons, WLANs re-define when devices are allowed to transmit. Instead of transmitting without regard to transmissions in other cells, 802.11 WLAN devices determine if the shared wireless medium is quiet and only transmit when this is so.
For example, for 2G systems, a frequency plan with adjacent co-channel APs is fatally poor. Handsets can transmit at the same time, and when they do, they create interference for one another so that the base-station cannot recover their data.
WLANs should ideally minimize the number of interfering devices, especially co-channel APs, that can detect each other, not the total interference power.
In summary, intuitive heuristics, developed from such fields as 2G cellular telephony, may be misleading when applied to WLANs. That is, the closest distance between two co-channel APs is less important than the total number of interfering APs.
Thus there is a need in the art for a method and system and software to derive a radio plan for a WLAN in a relatively straightforward and rapid fashion.